Centrifugal pumps can be used to convey fluids, such as water, oil or chemicals, and can be, for example, installed as water pumps in the piping network of either a drinking water transmission or distribution system, or a district heating distribution system or a cooling system in a power plant. Depending on the flow rate within the piping network, the number of centrifugal pumps can be determined to be operated in parallel. Accordingly, the number of active parallel centrifugal pumps varies with varying state of the piping network. In addition, the flow rate may be further controlled by adjusting the speed of the active parallel centrifugal pumps via a variable speed controller. Instead of water, for example, other mediums in fluid or gaseous form may be transported by the centrifugal pumps.
Apart from the flow rate, further control objectives for controlling the operation of parallel centrifugal pumps may be defined. For example, in US 2003/0235492 A1, a method is described for controlling a set of centrifugal or axial pumps in a manner that can reduce the chance of any pump operating in a zone in which damage or destruction can occur, which can be achieved by operating the pumps above their respective minimum flow limit, which is called Minimum Continuous Stable Flow (MCSF).
Another control objective can be the saving of energy. In the article “Parallel pumping, IPC technology in parallel pump control” by Li Jinguo, ABB Review Special Report Dancing with the Dragon, November 2008, pp. 77-81, energy savings can be achieved when the flow rate is controlled not via valve settings, as is the traditional method, but via a variable speed drive attached to each pump individually. In the traditional methods, the valve settings can be changed so that the point of operation of the overall system, including pumps, valves and pipes, is shifted from one so called system curve to another, where the system curves are shown as dependency of pressure drop H from flow rate Q and are depicted in the same diagram as the so called pump performance characteristics, which are also functions H=f(Q). In the variable speed control method described in the article, in addition to controlling the flow rate, the variable speed drives operate their corresponding pump at a pre-determined efficient speed which is determined to be lower than the traditional operating speed, while at the same time maintaining the overall efficiency at approximately the same level. When multiple pumps are operated in parallel, one of the corresponding drives can be designated as master drive and all other drives of the parallel pumps follow the reference of the master drive.